This invention relates to low-voltage space thermostats which control operation of heating-only systems and of heating and cooling systems.
U.S. Pat. No. 4,898,229 shows an electronic thermostat adaptable for use with a single-transformer or a two-transformer power source in a heating and cooling system. A single-transformer power source results in a system having four wires connected to the thermostat; a two-transformer power source results in a system having five wires connected to the thermostat. A disadvantage of this referenced thermostat is that it has only four wiring terminals. Specifically, when the installer of this referenced thermostat encounters the condition wherein the existing wiring to the thermostat location consists of five wires, he may not be sure as to how to properly connect the five wires to the four wiring terminals. The installer may conclude that the four-terminal thermostat is simply not the proper thermostat for use with five wires and return the thermostat to the seller, thus resulting in inconvenience, expensive service calls, and/or loss of sales.
It is desired to improve the referenced thermostat by providing five wiring terminals instead of four. The basic concept of a five-terminal thermostat being adaptable for use in a single-transformer (four connecting wires) or a two-transformer (five connecting wires) system has been known for many years. Such a five-terminal thermostat is shown in U.S. Pat. No. 4,308,991. Briefly, in such a construction, two of the thermostat terminals are connected together at the terminals by a removable wire jumper. When the heating and cooling system uses a single transformer, the wire jumper is retained, and one end of the secondary winding of the single transformer is connected to one of the two jumper-connected terminals. The other end of the secondary winding is connected through a fan relay, gas valve, and contactor to the remaining three terminals. When the heating and cooling system uses two transformers, the wire jumper is removed, and one end of the secondary winding of one of the transformers is connected to one of the two terminals previously connected by the wire jumper, and one end of the secondary winding of the other transformer is connected to the other of the two terminals previously connected by the wire jumper. The other end of the secondary winding of one of the transformers is connected through the gas valve to one of the three remaining terminals, and the other end of the secondary winding of the other transformer is connected through the fan relay and contactor to the remaining two terminals. Apparently, the five-terminal construction is sufficiently well known, especially by professional installers, so that no particular confusion exists when connecting such a five-terminal thermostat to either four or five wires.
The thermostat shown in U.S. Pat. No. 4,308,991 uses a mechanical system selector switch which provides for electrical isolation of the two transformers, from each other, in a two-transformer heating and cooling system. The thermostat shown in U.S. Pat. No. 4,898,229, of which the thermostat of the present invention is an improvement, uses electronic means rather than a mechanical switch, to effect the system selector function, and furthermore, embodies a common terminal to which both transformers are connected. It is noted that a modified construction of the thermostat shown in U.S. Pat. No. 4,898,229, such modified construction being described at column 8, line 53 through column 9, line 36 therein, provides for a five-terminal construction wherein electrically operated means is provided for effecting the system selector switch function, and the two transformers are electrically isolated from each other. However, such construction requires the addition of a relay which is relatively expensive.